This setting “Enable/Disable multiple logins into your account”, can be found in “Account Settings” under the “Account” tab.

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If enabled will allow multiple sessions to be opened into your account at the same time, meaning you will be able to log into your account, at the same time, from different places and browsers.

By default, this setting is disabled, meaning that if you log into your account from one place, and then log into the same account from another place, the first place you’ve logged in from will be logged out.

When you disable this setting (assuming that at some point you have enabled it), all other sessions other than your current session (if any exist) will be logged out.